This invention relates to hydraulic bolt tensioners.
In order to ensure that a nut on a bolt interconnecting two or more components is tightened to a desired degree, it is established practice to utilize a hydraulic bolt tensioner to extend the bolt within its elastic limit and to screw the nut up to the associated abutment surface of the component being tightened while the bolt is so tensioned such that, on release of actuating pressure within the hydraulic piston-cylinder assembly of the tensioner, a secure connection is effected.
Such hydraulic tensioners commonly incorporate a reaction member in the form of a nut or a puller sleeve screwed onto the free end of the bolt, the piston of the hydraulic piston-cylinder assembly reacting between the components to be interconnected and the reaction member to stretch or tension the bolt.
When the tensioning operation has been completed and prior to subsequent use of the tensioner, it is necessary to return the piston of the piston-cylinder assembly from its extended, operative position to its rest position.
It is known to provide hydraulic bolt tensioners incorporating labor saving means for automatically returning the piston to its rest position on release of hydraulic pressure thereon, such means comprising a spring reacting between the cylinder and the piston and constantly urging the piston towards its rest position. Thus, on release of hydraulic pressure on the piston, the spring returns the piston to its rest position and at the same time displaces hydraulic fluid back to a reservoir.
However, with such an arrangement, the time taken for the piston to be returned to its rest position can be substantial, the rate of return depending upon the spring load, the volume of hydraulic fluid to be displaced back to the reservoir, restrictions in the connecting hoses and the like.
In certain situations, such as sub-sea and in nuclear installations, operating time is of prime importance and the slow speed of return of the piston of these known bolt tensioners can create problems.